In the electronic industry field, ultrapure water is used for cleaning water of materials and parts. In particular, the quality of ultrapure water used in a semiconductor manufacturing factory or a wafer manufacturing factory is strictly controlled, for example, resistivity: 18.2 MΩ·cm or more, number of particles having a diameter of 50 nm or more: one/mL or less, number of bacterium: one/L or less, TOC (total organic carbon): 1 μg/L or less, ionic silica: 0.1 μg/L or less, metals: 1 ng/L or less, and ions: 5 ng/L or less.
The place where ultrapure water is used (point of use) is connected to an ultrapure water manufacturing apparatus and ultrapure water supply piping (channel), and residual ultrapure water that is not used at the point of use returns to the ultrapure water manufacturing apparatus through another ultrapure water supply piping, resulting in formation of a circulatory system to configure an ultrapure water manufacturing system as a whole.
Conventionally, cleaning with alkaline solution is used when an ultrapure water manufacturing system is newly or additionally set, modified, or maintained so that particles such as dust, silica, and aluminum in the air that exists into the system, particles included in water, such as remains of bacteria and rust, refuses which occurred during the system production, such as a filter, piping and more (hereinafter, these are collectively referred to as “particles”) are properly eliminated to the outside of the system, and the number of particles having a diameter of 50 nm or more in one milliliter of ultrapure water is one or less (for example, Patent Document 1).
In addition, a method of sterilizing an ultrapure water manufacturing system in order to suppress bacteria in ultrapure water is proposed (Patent Documents 2 and 3).
Furthermore, a method of cleaning and sterilizing an ultrapure water manufacturing system by cleaning the inside of the ultrapure water manufacturing system with alkaline solution and then with hydrogen peroxide solution is proposed (Patent Document 4).
Incidentally, the principle of removing particles with the alkaline solution is as follows.
The adhesion of particles to piping and others of an ultrapure water manufacturing system is electrical, i.e., electrostatical due to the surface potential. In general, the surface potential of particles in solution such as cleaning fluid varies depending on the solution characteristics and, particularly, can be significantly changed by varying the pH of the solution. A shift in the pH of the solution to the alkaline side makes the particles negatively charged and also makes the charge increase. However, no change in surface potential of an organic polymer material, such as PVC (polyvinyl chloride) or PPS (polyphenylene sulfide), forming the piping system and the like of the ultrapure water manufacturing system is caused, and the material has negative charge regardless of the pH change of the fluid in contact therewith. Therefore, the particles negatively charged by shifting the pH of the fluid in contact therewith are electrically repelled from the material forming the system and are easily peeled and removed.
Furthermore, the peeling and removing action can be sufficiently achieved even if the concentration of alkaline solution used as the cleaning fluid is low (for example, several tens milligrams per liter). Therefore, the cleaning fluid can be used at a low concentration. Consequently, the ratio that a component of the cleaning fluid remains in the system is reduced, and the increase of TOC by cleaning fluid is inhibited. As a result, the cleaning work can be finished within a short period of time, and the build-up of the quality of ultrapure water produced by the ultrapure water manufacturing system is possible.
Since the method for cleaning an ultrapure water manufacturing system with alkaline solution is excellent for removal efficiency of particles, particles adhering in the ultrapure water manufacturing system can be promptly peeled and removed. In addition, since the concentration of the cleaning fluid is low, the component of the cleaning fluid hardly remains after cleaning, and thereby TOC is hardly increased. Therefore, the cleaning work can be performed within a short period of time.
In Patent Document 4, efficient cleaning and sterilizing is possible by the cleaning and sterilizing method in which hydrogen peroxide is further injected for sterilization after the cleaning with alkaline solution.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-317413
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-166283
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-275881
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-192162